High dose vitamin B12 supplements can sometimes cause side effects, including headache, nausea, vomiting, mild diarrhea, itching, skin rash, tiredness, weakness, or tingling in the hands and feet. OC: Why did B12 make me feel worse?, I hear this all the timewomen tell me they started B12 shots or IVs and suddenly felt overstimulated, jittery, or couldnt sleep., Heres whats really going on: Intramuscular Injections B12 MIC Injections Online Boost Energy & Metabolism B12 Injection Dosage and Frequency: 7 Essential Guidelines for Adults Liv Hospital